OBJECTIVE: To investigate the extent to which defects in the human sperm centrosome are causes of male infertility. RESULTS Imaging techniques have demonstrated that human sperm can be induced to nucleate microtubules in vitro in a cell-free Xenopus egg extract, and human sperm of varying fertility differs in its ability to nucleate microtubules. Successful human fertilization depends on flawless sperm functioning. This includes the sperm's activities before it meets the oocyte, sperm-oocyte plasma membrane fusion, and the cytoplasmic events mediated by the sperm nucleus and centrosome that conclude fertilization the merging of the parental genomes in the activated zygote. Defects in sperm behavior at any stage result in the inability to complete fertilization, and quantitative analyses of these defects have been developed into assays for male infertility. These assays determine the extent of aberrations in sperm number, morphology, motility, the ability to undergo the acrosome reaction, the release of acrosome enzymes, and penetration assays into zona-free hamster oocytes or isolated zonae. This application investigating idiopathic male infertility studies the microtubule organizing capacity of the human sperm centrosome. Our aims were as follows 1 Is centrin, a centrosomal protein, found in human sperm, and do its concentrations vary predictably in sperm donated from men of differing fertility? 2. Does the binding of maternal centrosomal proteins, especially -tubulin, vary predictably in sperm from men of differing fertility? 3. Are there differences in the ability of sperm from men with varying fertility to nucleate and direct the assembly of microtubule-containing asters in extracts from Xenopus eggs? 4. Will the sperm asters formed in pronucleate-stage mammalian oocytes, inseminated with sperm from men of varying fertility, predictably vary in their size and microtubule density? FUTURE DIRECTIONS We plan to explore the possibility of developing a simple assay to test for sperm microtubule nucleating ability in vitro. KEY WORDS human, centrosome, sperm, motility, cell-free extract, Xenopus